feat: apply transfer converters to System Nexus

[tconley1428]

Summary

• scopes the System Nexus payload/failure converter context while serializing and deserializing outer System Nexus envelopes
• uses a local unreleased nexgen build to generate .NET transfer-type converters and context-backed support helpers
• generates a payload visitor for payload-bearing Workflow Service envelopes; any payload marked as a System Nexus envelope always bypasses the user codec

Status

Work in progress — do not merge yet. This PR must wait for the nexgen changes in temporalio/nexgen#128 to be released, then replace the local-generator integration with that released version.

There is also a server dependency: System Nexus result envelopes are not currently marked by the server. Until that behavior exists, codec coverage for marked result envelopes cannot be enabled; the end-to-end Signal-with-Start test intentionally uses no payload codec.

Validation

• dotnet test tests/Temporalio.Tests/Temporalio.Tests.csproj --no-restore --filter 'FullyQualifiedName~SystemNexusPayloadConverterTests|FullyQualifiedName~SystemNexusTests'\n- dotnet format --verify-no-changes --no-restore (blocked only by pre-existing unused imports in tests/Temporalio.SimpleBench/Program.cs)\n- dotnet build --no-restore (blocked because tests/Temporalio.SimpleBench/obj/project.assets.json is absent)